Air entering a compressor under various relative humidity conditions is compressed on the order of 9.5 atmospheres. The compression process heats the air and allows the air to hold all the moisture (water vapor) it can contain. As air is compressed, its temperature rises rapidly and with each rise of about 20.degree. F. its capacity to hold moisture almost doubles. However, as the air is compressed to a smaller and smaller volume, its ability to hold moisture decreases at approximately the same ratio of the increase of its ability to retain moisture with temperature increases. Thus, the greatly reduced volume of air finally leaving a compressor can have a temperature of about 400.degree. F. to 500.degree. F. and a relative humidity quite low at that temperature and reduce volume.
On locomotives, the compressor supplies pressurized air to one or more reservoirs or reservoir tanks for operating the air brakes of the locomotive and any railway cars connected to the locomotive. As the air enters the reservoirs from the compressor its temperature drops thereby reducing the moisture carrying capacity of the air while simultaneously expanding to increase this capacity. Any water laden air entering the reservoir and then supplied to the brakes of the locomotive and railway cars can be the cause of a substantial number of harmful effects in regard to brake hardware and equipment of the locomotive and cars. For example, such water laden air can wash away lubricants, cause rusting and excessive wear, all of which results in increased maintenance and maintenance costs.
The problems are made even more critical when the moisture freezes thereby effecting the ability of the brakes to operate at all as well as other safety devices on the locomotive using pressurized air such as windshield wipers, defrosters, bells and horns.
U.S. Pat. No. 5,106,270 to Goettel et al discloses a locomotive air compressor provided with an integral aftercooler (and integral intercoolers) that is effective to lower the temperature of the pressurized air leaving the compressor to that approaching atmospheric ambient. The disclosure of the Goettel et al Patent is incorporated herein by reference.
The benefits of the Goettel et al integral aftercooler are known in the railroad industry, yet not all locomotives are so equipped. Locomotives without such aftercoolers are in need of aftercoolers for the reasons discussed above.
An aftercooler uses a fan to pull or push outside ambient air past finned heat exchanger tubes receiving compressed air from the compressor. Since outside ambient air can be at a level that produces freezing of the moisture laden air in the aftercooler and in its discharge piping that feeds the locomotive reservoirs, there is a need to prevent the occurrence of freezing when the ambient temperature is at or below freezing.